"Terrorism does remain the FBI's top priority, but in the not too-distant-future we anticipate that the cyberthreat will pose the greatest threat to our country," he said.

"Today, terrorists have not used the Internet to launch a full-scale cyberattack, but we cannot underestimate their intent."

According to Mueller, the FBI maintains a dedicated cyber-security squad in each of its 56 field offices. In addition, 1,000 agents and analysts are currently focused on countering threats posed by terrorists, organized crime rings and state-sponsored cyber-espionage.

"State-sponsored hackers are patient and calculating. They have the time, money and resources to burrow in and wait. You may discover one breach only to find that the real damage has been done at a much higher level.

"There are only two types of companies: those that have been hacked, and those that will be. Even that is merging into one category: those that have been hacked and will be again. Maintaining a code of silence will not serve us in the long run. For it is no longer a question of 'if' but when and how often," he added.